There are many different types of raingages in use around the world today, the basic design being an open-topped cylindrical vessel standing substantially upright on the ground. The orifice of the vessel serves as a receiver for rain, with water falling into the orifice being transmitted into the vessel for storage and measurement.
For example, the United States National Weather Service (USNWS) standard raingage for measuring falling rainfall consists of an open-topped cylinder with an 8-inch wide orifice for receiving rainfall, a funnel to direct water from the orifice receiver into a measuring tube, and a storage vessel to catch water overflowing from the tube. The measuring tube has a cross-sectional area of approximately 1/10 the width of the storage vessel's orifice. Thus, 0.01 measured inches of rain will be 0.1 inches deep in the measuring tube as read directly from a calibrated measuring stick. The measuring tube is 20 inches in height and holds exactly 2 inches of rain. Any additional rainfall will flow into the outer storage vessel (see FIG. 1a). The gage is used in a substantially upright, vertical position, with the orifice being about a meter or less above the ground.
The USNWS standard gage measures rainfall by quantifying the volume of water falling into the raingage orifice in a downward vertical trajectory. However, the air is generally not calm during stormy weather, and the presence of a raingage on the ground interferes with local wind movement, distorts the local wind field, and creates eddies and turbulence around the gage orifice and its immediate surroundings. The local air disruption causes some of the raindrops which would otherwise fall into the gage to instead strike the gage at inclined angles, thus rendering the effective orifice catch size smaller than its actual dimensions. As a consequence, point precipitation measurement is always deficient when evaluated with respect to the actual incidence of local precipitation.
Errors in precipitation measurement due to, for example, the raingage being unlevel, the color of the gage, water splashing out of the gage's orifice, evaporation, and/or adhesion of the water to the walls of the gage, are generally less than .+-.1% each, but local wind effects can cause measuring errors as large as from about 5% to about 80%. The water catch deficiency due to wind effects is a combined function of both the horizontal wind speed and the intensity of the rainfall. As rainfall intensity decreases, raindrop diameters decrease and the terminal velocities of individual raindrops decrease, while angles of raindrop inclination increase for a given wind speed. Thus, negative wind effects on measured precipitation are greater for light storms than for heavy storms.
There have been many studies conducted to improve precipitation measurements. Techniques and devices suggested for improvements include proper measuring site selection (Leonard and Reinhart, 1963; Chang and Lee, 1975; Golubev, 1985; Sevruk and Zahlavova, 1992); the use of shielded gages (Nipher, 1878; Alter, 1937; Warnick, 1953; Lapin and Samaj, 1989), tilted gages, (Hamilton and Reimann, 1958), pit gages (Koschmeider, 1934; De Bruin, 1985) and dual gages (Hamon, 1971; Larson and Peck, 1974; Rawls et al., 1975); computational corrections for wind effects (Chang and Lee, 1974; Chang and Lee, 1975; Allerup, 1985; Gronowski, 1989), and the use of lysimeters (McGuiness, 1966; Morgan and Lourence, 1969) and vectopluviometers (Hamilton, 1954). Each of these gages and measuring techniques address the problem of wind effects on precipitation measurement, and purport to reduce the effects of wind on the accumulation of precipitation to a minimal level. Each of these approaches, however, has proven deficient in practice.
For example, the pit gage, or so-called sunken gage, is a standard gage installed in a conical pit of 1-2 m. in diameter, with an orifice located at ground level. Since wind speed increases with height above the ground, pit gages are probably the most accurate in point precipitation measurements. They are generally accepted by those of skill in the art to be the least biased and truest means of measuring local precipitation, and are frequently used as a reference gage for calibrating other types of gages. However, pit gages have proven to be wholly inadequate for measuring snowfall, are difficult and expensive to use for large-scale applications, and are easily interfered with by litter, animals, falling leaves and other orifice blocking materials.
Since wind fields behave in a vector-like manner, they change speed and direction with respect to time and space. The great variation of wind patterns and speeds renders each of the known techniques and methods discussed above scientifically unacceptable (ie., the margins of measuring error are unacceptably high), and improvements thereto are generally found to be minor or insignificant in practice. Therefore, the effects of wind dynamics on precipitation measurement are still a major concern in modem precipitation studies.